Electroluminescent-photoconductor devices having improved input-output isolation



T. E. BRAY ETAL ELECTROLUMINESCENT-PHOTOCONDUCTOR DEVICES HAVING IMPROVED INPUT-OUTPUT ISOLATION Filed Nov. 7, 1961 Dec. 29, 1964 I2 ELECTRODE I3 ENVELOPE ll ELECTROLUMINESCENT I PHOSPHOR 5 PHOTOCONDUCTOR 4 ENVELOPE 3 ELECTRODE 2 ELECTROLUMINESCENT PHOSPHOR 2| TRANSPARENT FIG.IA

THEI AGENT.

3,163,763 ELECTRQLUh IEQESQENT PHQTGCONDUCTOR DEVICES HAVlNtG HVEPROVED INPUT-GUT- PUT KSGLATIGN Thomas E. Bray, Clay, and Richard D. Stewart, Syracuse,

N.Y., assignors to General Electric Company, a corporation of New York Filed Nov. 7, 1961, Ser. No. 150,713 6 Claims. (Cl. 250-266) This invention relates to improvements in optoelectronic devices comprised of pairs of electroluminescent and photoconductor elements which are suited for a wide variety of applications such as general switching functions, amplifiers of light or electrical energy, indicators, etc. The invention provides a construction of an electroluminescent-photoconductor pair in which there is substantially no feed-through from the input signals applied to the electroluminescent element to the output circuit of the photoconductor element. The novel construction is particularly advantageous for switching circuits which require low noise operation.

In switching applications, electroluminescent-photoconductor devices are generally four-terminal devices having a pair of On/Otl output terminals which function as a SPST switch which is controlled by signals appearing at the pair of input terminals. The four-terminal devices are comprised of a photoconductor intermediate the output terminals and an electroluminescent element intermediate the input terminals which is arranged to illumimate the photoconductor. Accordingly, when a suitable energizing signal is applied to the input terminals, the electroluminescent element radiates light which illuminates the photoconductor, reducing its resistance, and efiectively closes the circuit between the output terminals. in the absence of an energizing signal applied to the input erminals, the circuit between the output terminals is effectively open.

An electroluminescent element is here defined as an element which includes means for producing an electric field and an electroluminescent-phosphor material which is made to luminesce or emit radiation under the stimulus of the applied electric field. For a survey and bibliography of the phenomenon of electroluminescence, reference is hereby made to an article entitled Electroluminescent and Related Topics, by Destriau and Ivey in the Proceedings of the IRE, December 1955. p

A photocouductor element is here defined as an element having a pair of terminals bridged by a material the impedance (generally the resistance) of which varies as a function of the illumination due to radiation emitted by the particular associated light source. The nature and properties of both types'of materials selected for the elements determine the electrical characteristics of the device.

Generally, electroluminescent elements radiate mostly visible light and most photoconductor elements are responsive to substantial portions of the visible light spectrum. However, it will be understood that the term United States Patent luminescent-photoconductor device adapted for printed the spectrum. The essential factor is that the active 3,163,763 Patented Dec. 29., 1964 for the light coupling effect. On this basis, electroluminescent-photoconductor switching devices can control very low level signals. The lower limit should only be fixed by the inherent noise level of the output photoconductor circuit which can be as low as on the order of 10 microvolts. As a basis of comparison, it should be noted that signals controlled by transistor switches generally have a lower limit of about 1000 microvolts. In practice, however, it has been found that the electrical isolation of optoelectronic circuits is frequently unsatisfactory.

Practical structures are generally formed with the electroluminescent and photoconductor elements in very close physical proximity to provide compactness, optimum light efiiciencies, ease of fabrication, etc. It has been discovered that this proximity results in undesired signal feed-through between input signals applied to the electroluminescent element and the output circuit which includes the photoconductor element.

Photoconductors generally exhibit a response time lag which results in their maintaining the low resistance state for a finite time after removal of illumination. Although this characteristic places some restrictions on switching speeds, it is very advantageous to input-output isolation in the following respect. Most practical electroluminescent elements require an A.-C. excitation signal and exhibit a variation in brightness at twice this excitation frequency. This brightness variation should produce a corresponding variation in the photoconductor resistance which modulates the output in accordance therewith. Such a coupling of the input excitation signal at twice the input signal frequency is not observed. It is believed that the photoconductor response time lag masks this modulation.

However, the input signals required to excite the electroluminescent elements are of large amplitude (being typically volts) relative to very low level signals in the output. It has been observed that these input signals, with their close proximity to the photoconductor elements, produce sufiiciently large electric fields to cause substantial pick-up by the photoconductor element in the output circuit. It has also been observed that input signal currents cause signal pick-up in the output circuit by means of the associated changing magnetic fields, although this effect is much smaller than the electric field effect.

It is therefore an object of the invention to minimize signal feed-through in an optoelectronicdevice.

It is a further object of the invention to substantially eliminate the effects of field coupling between an electroluminescent element and a photoconductor element in an optoelectronic device. g

It is a still further object to provide a compact electrofabrication in whole or in part and which maintains etfective input-output isolation.

It is another object of the invention to provide a compact electroluminescent-ptoconductor switching device for low level signals in which signal feed-through due to electric and magnetic field coupling between the input and output circuits is effectively eliminated.

Briefly stated, in accordance with one'aspect of the invention, the effects of undesired field coupling between electroluminescent and photoconductor elements are .sub-

stantially eliminated by providing a structure in which the electric and magnetic fields cancelto produce a neutralized eifect. Such a construction utilizes two electroluminescent elements which are symmetrically disposed about the photoconductor element. The coupling of the input signal to the electroluminescent element is then arranged to produce signalsin the electroluminescent'elements which are of opposite phase to produce a cancelling relationship in the photoconductor.

The features of the invention which are believed to be glass or other rigid electrical insulatingmaterial.

constructed in accordance with the invention.

FIGURE 1A is a diagram of the electrical circuit or the FIGURE 1 device.

FIGURE 18 is a diagram illustrating the electric fields Referring now to the drawing and inparticular to FIGURE/1 thereof, there is illustrated an exploded'sectional view in perspective of an electroluminescent-photoconductor device of the present invention. The device is supported. by a planar base plate 15 which is conveniently The optoelectronic device is formed by arranging successive electroluminescent and photoconductor elements with specific geometrical relations and specific means for energization. In the embodiment of FIGURE 1, each element is preferably formed as successive layers upon a substrate by techniques such as those employed for printed circuits or analogous automated processes. The device is then formed by assembling the elements upon base plate 15 in a sandwich-like configuration.

A first planar electroluminescent element 32 is disposed upon base plate 15. This element is conveniently formed upon a thin substrate 16 suchas glass. Deposited upon and in extended area contact with substrate leis a light reflective conducting electrode 1 which can be an opaque thickness of aluminum or silver, for example. Deposited upon and in extended area contact with electrode layer 1 is a layer, 2 comprised of a powdered crystalline mass of electroluminescent phosphor suspended in a dielectric me- I dium, or one or more properly aligned single crystals of an I electroluminescent material. Conveniently, the phosphor may be zinc sulphide activated with about 0.3 by weight of copper and dispersed in a transparent dielectric. It will,

of course, be understood that these examples are given merely by way of illustration, .and that any suitable electroluminescent phosphor can be used. Conveniently,

electroluminescent layer 2 can be deposited by spraying a light-transmitting dielectric medium which can, for example, be nitrocellulose or an alkyd resin in which the Alternatively, a very thin, light-transmitting layer of evaporated material such as aluminum or silver may be used. A thin, transparent plastic envelope 4 forms a hermetically sealed encapsulation about electroluminescent element 32.

l A planar photoconductor element 35 is disposed over electroluminescent element 32 and is coextensive therewith. This element is conveniently formedupon a thin substrate 17 such as glass. Deposited upon 'the substrate 17 is a photoconduct'or electrode 6 which is conveniently formed of platinum or indium 'in a manner generally similar to the electroluminescent electrode layer 1. How

ever, the photoconductor electrode 6 is fabricated with J "two separated portions, 6A and 6B, which form two terminal electrodes. The electrode portions 6A and 6B are preferably in the form of an interloclging comb-like geometry as illustrated which provides long perimeters that are uniformly spaced. Deposited upon and extending' through the spaces in the photoconductor electrode structure ;6 is a layer of photoconductive material 5. This 'material is suificiently thin to be translucent and can,

for example, be comprised'of cadmium sulfide or lead sulfide which can be sintered, sputtered, or evaporated on the electrodes 6A and 6B. More generally, photoconductive layer 5 can, for example, consist of any of the sulfides, selenides, or tellurides, of cadmium, lead or zinc, or can be any other photoconductor deposited in any conventional manner. The photoconductor element 35 is hermetically sealed by a thin, transparent plastic envelope 8 which, together with the envelope 4 of electroluminescent element 32, electrically insulates the two elements.

A second planar electroluminescent 31 is placed over photoconductor element 35 and is coextensive therewith. Element 31 is formed as a mirror image of electrolumincs cent element 32. Upon a glass substrate 18, successive layers form an electrode layer 10, an electroluminescent phosphor layer 1L'and an electrode layer 12 which are fabricated in the same manner as layers 1, 2 and 3, respectively. The element 31 is then hermetically encapsulated by an insulator envelope 13 in the same manner as electroluminescent element 32. p

The electroluminescent elements are energized in the following manner. A pair of input terminals 20 are connected to a transformer 21 which provides two signals which have opposite phases. The output center tap terminal 22 is connected to a source of reference potential which serves as ground and to a pair of terminals 23 on the electroluminescent electrode layers 3' and 16. The remaining terminals of the output winding of transformer 21 are connected to terminals 24 and 25 on respective electroluminescent electrode layers 1 and 12. The output terminals 29 of the device are connected to respective portions 6A and 6B of the photoconductor elec' trode 6.

The operation of the device illustrated in FIGURE 1 may be considered in connection with FIGURE 1A which is a diagrammatic representation of the electric circuit of the device. The photoconductor element 35, comprised of layers 5 and 6, when DARK (not illuminated by the electroluminescent element) presents a relatively large 'impedence between the output terminals 29 which is effectively an open circuit. When an input signal is applied across the terminals 20, the transformer 21 divides the signal into two parts which have a phase differential of 1r radians. These portions of the signal are applied to the electroluminescent elements 31 and 32 which are comprised respectively of layers 1-3 andltl-IZ. While an input signal is applied across the electroluminescent ele ments, light is radiated which illuminates the photoconductor 35. When the photoconductor element is illuminated, it is in a LIT state having a low resistance which is effectively a closed circuit between the output terminals 29f The light coupling between each electroluminescent element and the photoconductor element operates in the usual manner to controlzthe impedance state of photoconducor 35., However, because each electroluminescent element 31, 32 is energized by signals which are 11' radians out of phase, the voltages induced in the photoconductor by field. coupling have opposite polarities and therefore.

cancel.

The electric fields produced by the electroluminescent elements 31 and 32 are illustrated in FIGURE 13. Because of the nature of 'theseclements, large electric fields 51 and 52 exist betweenthe electrode layers to produce ing between the electroluminescent elementswill be present inthe photoconductor element. Accordingly, .there will'be an effective capacitive coupling of the input signal to the output terminals through this stray capacitance.

However, because of the 7r radians phase differential-in the signals applied to elements 31 and 32, the fields largely cancel. The neutralizing effect of the fields is in fact However, there will also be substantial 1 5 dependent upon the phase differential which must be within a fraction of a radian of ar radians differential.

There will also be currents flowing in the electrode layers 3 and of the electroluminescent elements 31 and 32 which will produce magnetic fields. These magnetic fields will induce currents in the photoconductor 35. L1 practice, it has been found that there are wide variations in the kinds of current distributions encountered which depend primarily on the type of device geometry. However, the net effects are usually small relative to the electric field. When they are large, their effects can be substantially neutralized by terminal placement which produces a neutralizing effect.

In considering the efiects of electric field-coupling as illustrated in FIGURE IE, it should be noted that it is insufiicient to have merely opposing electric fields produced by the electroluminescent elements. The essential requirement is that the fields must be neutralized. In the FIGURE 1 embodiment, this is obtained by substantially grounding electrodes 3 and It) and producing identical charge distributions (of opposite polarity) along the electrodes. The FIGURE 1 device would not provide sufficient input-output isolation if both the electroluminescent elements were driven in phase but with electrodes 3 and 10 connected to different input terminals because these electrodes and the photoconductor therebetween would form a capacitor-like structure which would produce substantial signal feed-through.

The achievement of the desired input-output isolation is therefore dependent upon the overall neutralizing capacitor configurations. The neutralization results from (1) providing a capacitor structure which has mirror symmetry about the photoconductor element and (2) connecting the electroluminescent element electrodes nearest the photoconductor element to a source of reference potential. Since only one electroluminescent element is required to illuminate the photoconductor element, it is evident that the FIGURE 1 device can be modified by replacing one electroluminescent element' with a capacitor element which produces the same field effects as an electroluminescent element. For example, an element formed in the same manner as an electroluminescent element but Without the electroluminescent phosphor addition to the dielecsimplifiies the formation of a device with mirror symmetry.

In a representative realization of the FIGURE 1 device, the device area is 0.25 square inch. Typical input signals are 400 cycles per second at 30 microamperes and 100 volts. Without the field neutralization construction, undesired induced signals in the photoconductor are-frequently several hundred microvolts. However, the embodiment of FIGURE 1 provides a reduction of the induced voltages by an order of magnitude. If conventional shielding techniques are employed such as the insertion of transparent grounding planes betweenthe electroluminescent and photoconductor elements, additional improvement in performance is obtained. The undesired induced voltages will be reduced by another order of magnitude, but with some loss of light efiiciency.

Obviously, there are wide areas of variation in the construction of an electroluminescent-photoconductordevice in accordance with the invention. It is frequently desirable to construct arrays of optoelectronic devices and the array element are conveniently formed simultaneously on common substrate members. The device of FIGURE 1 is constructed with a single pair of input terminals and a single pair of output terminals. In an application Where a plurality of isolated input circuits and/or a plurality of isolated output circuits are required, they can be provided in a single device. As one example, the photoconductor elements can be separated into two isolated and symmet rical halves which have their own pairs of terminals.

Such a construction provides a DPST switch. Similarly,

and photoconductor elements are then separated by trans- 1 7 parent dielectric layers and the whole device is encapsulated with a single envelope.

Some other variations in construction include the use of difierent geometries such as cyclindrical surfaces and layers, Also,'some modifications can be made if auxiliary light output signal are desired so as to provide a visual indication of switch state, for example. For this purpose,

one or both of the exterior electrodes 1 and 12 are made light-transmitting. When transparent, these electrodes are made in the same manner as electrode layers 3 and 10. However, when light-transmitting exterior electrodes are employed rather than reflective ones, it is necessary to allow for the light loss in respect to the photoconductor element illumination.

Also, the FIGURE 1 device is illustrated as employing a photoconductor element in which the photoconductor layer 5 is not much thicker than the electrode layer 6. With this arrangement, the photoconductor layer 5 is sufiiciently light-transmitting so that the light from both electroluminescent elements reaches a sufiicient depth in the photoconductor layer to produce a low resistance path between =the electrode portions 6A and 6B. However, since the photoconductor materials have poor light-transmis sion properties, it is frequently advantageous to have the photoconductor electrode extend the width of the photoconductor element with photoconductor material only in the space between the electrode portion 6A and 6B.

In the FIGURE 1 illustrated form of photoconductor 35, electrode layer 6 is not equally spaced from both electroluminescent elements. However, it has been found that the field neutralization in the photoconductor element region is sufficiently complete so as to remove any deleterious effects. However, if this spacing differential, or

duced by providing unequal excitation of the individual electroluminescent element. This can be obtained, for example, by providing differential attenuation of the signal portions applied to the individual electroluminescent elemerits or by unbalanced windings in transformer 21. The

essential requirement that there be mirror symmetry at the photoconductor element is then met.

In regard to transformer 21, it is noted that device performance can frequently beimproved if the reactance of the transformer is selected to. produce series resonance withxthe capacitance of the electroluminescent elements in order to reduce the loading on the input signal source.

The function of the transformer 21 is to divide the input trative embodiments, it is to be understood that all modifications, substitutions and omissions obvious to one skilled'in the art are intended to be within the spirit and scope of the invention as defined by the following claims.

.What is claimed is: 7 1. An electroluminescent-photoconductor device in a sandwich-like configuration with improved input-output isolation comprising:-

(a) a planar photoconductor element;

(b) a planar electroluminescent element arranged to illuminate said photoconductor element;

(c) a planar capacitor element arranged proximate said photoconductor element with mirror symmetry relative to said electroluminescent element and; having a construction which produces an electric field pattern similar to that of said electroluminescent element; (d) input means to derive substantially equal and out of phase voltages from input signals; and (e) means coupling said input means to both said electroluminescent elementrand said capacitor element to produce excitation fields which are neutralized in said photoconductor element. 2. A11 electroluminescent-photoconductor switch in a sandwich-like configuration with improved input-output isolation comprising: I t (a) a planar dielectric support member; (b) a first planar electroluminescent element arranged on said support member and including a pair of symmetry in respect to and substantially coextensive with said first electroluminescent element; p (e) means to ground the electrodes of said electroluminescent element nearest the photoconductor element;

t (1'') an input transformer connected to said electrodes of said firs-t and second electroluminescent elements to couple portions of applied A.-C. input signals to said elements having opposite phase to provide electric field neutralization in said photoconductor element. 3. An electroluminescent-photoconductor control device in a sandwich-like configuration with improved electrical isolation between the input and the output circuits comprising: 1

(a) variable resistance means for connection to an output circuit to effect control thererofcornprising a planar photoconductor element whose conductivit is responsive to incident illumination; 7 (b) a pair of similar planar electroluminescent elements responsive to input electrical excitation arranged adjacent opposite surfaces of said photoconductor element and optically coupled thereto, each electroluminescent element including a pair of spaced planarelectrodes between which is disposed a layer of electroluminescent material;

. '8 (c) connective means interconnecting the electrodes "of said electroluminescent elements nearest said photoconductor element;

(dyinput terminal means for connection to a source of input excitation sig'nals; and

(e) means coupling said input terminal means to both said electroluminescent elements for excitation thereof. t Y

4. The electroluminescent-photoconductor control device of claim 3' wherein means are provided coupled to said input terminal means to derive substantially out of phase voltages from said source, and wherein said coupling means are poled to produce excitation fields which are neutralized in said photoconductor element.

5. The electrolurninescent-photoconductor control device of claim 3 wherein said connective means are connected to ground. 1 7

- 6. An electroluminescent-photoconductor control device in a sandwich-like configuration with improved electrical isolation between the input and output circuits comprising: t I p (a) variable resistance means for connection to an out put circuit to effect control thereof comprising av planar photoconductor element whose conductivity is responsive to incident illumination; 6

(b) a planar electroluminescent element responsive to input electrical excitation arranged upon and in close proximity to one surface of said photoconductor element and optically coupled thereto, said electroluminescent element including a pair of spaced planar electrodes between which is disposed a layer of electrolumirnnescent material;

(c) a conductive layer symmetrically arranged upon the surface of said photoconductor element opposite that of said electroluminescent element and connected' to the electrode of said electroluminescent element nearest said photoconductor element;

(d) input terminal means for connection to a source of excitation signals coupled to said electroluminescent element for excitation thereof, said connection of said planar electrode with said conductive layer effecting a reduction of the excitation field in said photoconductor element.

References Cited by the Examiner UNITED. STATES PATENTS G. NILSON, Primary Examiner.

' WALTER STOLVVEIN, Examiner. 

1. AN ELECTROLUMINESCENT-PHOTOCONDUCTOR DEVICE IN A SANDWICH-LIKE CONFIGURATION WITH IMPROVED INPUT-OUTPUT ISOLATION COMPRISING: (A) A PLANAR PHOTOCONDUCTOR ELEMENT; (B) A PLANAR ELECTROLUMINESCENT ARRANGED TO ILLUMINATE SAID PHOTOCONDUCTOR ELEMENT; (C) A PLANAR CAPACITOR ELEMENT ARRANGED PROXIMATE SAID PHOTOCONDUCTOR ELEMENT WITH MIRROR SYMMETRY RELATIVE TO SAID ELECTROLUMINESCENT ELEMENT AND HAVING A CONSTRUCTION WHICH PRODUCES AND ELECTRIC FIELD PATTERN SIMILAR TO THAT OF SAID ELECTROLUMINESCENT ELEMENT; (D) INPUT MEANS TO DERIVE SUBSTANTIALLY EQUAL AND OUT OF PHASE VOLTAGES FROM IMPUT SIGNALS; AND (E) MEANS COUPLING SAID INPUT MEANS TO BOTH SAID ELECTROLUMINESCENT ELEMENT AND SAID CAPACITOR ELEMENT TO PRODUCE EXCITATION FIELDS WHICH ARE NEUTRALIZED IN SAID PHOTOCONDUCTOR ELEMENT. 